1. Field of the Invention
This invention relates to a new and improved lighting feedback control system for applying AC power from an AC power source to at least one ballasted lamp.
2. Description of the Prior Art
Various prior art circuits have addressed the problems associated with controlling the level of light illumination in a room or in a portion of a domestic or commercial building. Typically, the prior art lamp control circuits were directed to maintaining constant lamp illumination, such as for example disclosed in U.S. Pat. No. 3,609,451 to Edgerly et al. In the Edgerly et al. patent, lamp energizing power is derived from an auto transformer having a variable tap coupled to a lamp load with the positioning of the tap being controlled by a motor feedback system in accordance with the illumination sensed by a light sensor.
Another control circuit disclosed in U.S. Pat. No. 3,684,919 to Cramer is directed to a dimmer circuit for controlling the light intensity from a lamp by adjusting the firing angle of a silicon controlled rectifier (SCR) or like control element supplying AC power to the lamp. The circuit includes a firing angle function generator which produces, in sync with each AC half cycle, a signal f(.alpha.) monotonically related in amplitude to SCR firing angle. Comparator circuitry triggers the SCR's when the signal f(.alpha.) crosses the level of a light intensity control signal linearly related e.g. to dimmer control handle position. In one embodiment, the function generator includes a capacitor charged at preselected rates during portions of each AC half cycle such that the firing angle function thereby synthesized is programmable to implement any desired dimmer response.
Yet another lamp control and switching circuit is disclosed in U.S. Pat. No. 4,135,116 to Smith, in which the illumination generated by a light source is monitored, and a feedback signal generated in order to produce a gaining control signal to a switch connected in series with the light source, by which the degree of illumination produced therefrom is controlled in correspondence to the sensed output thereof.
A serious drawback of many prior art light control systems arises due to the difficulty of adjusting lighting conditions for different locations to suit the lighting requirements of each location. A further disadvantage resides in the fact that the prior art lamp control systems also generally result in the application of full power to the lamp device being controlled upon initial application of power to the lamp. However, the predominate failure mode of ballisted lamps occurs upon full application of power to a cold cathode filament, since the cold filament exhibits low resistance which produces an initial surge current upon application of full power, often destroying the lamp. Yet another problem is frequently experienced, in that typical prior art systems energize a lamp load even when ambient lighting is otherwise adequate, thereby needlessly increasing energy consumption.
U.S. Pat. No. 3,898,516 to Nakasone addresses some problems associated with cold cathode lamp firing and proposes a "soft switching" technique in which the conduction angle of a lamp switch is gradually increased following initial turn-on in accordance with the resistance of a temperature sensitive thermistor connected in circuit with the lamp.